This invention generally relates to reworking circuit boards. The invention relates more specifically to de-soldering or removing components from, and attaching or soldering components to, circuit boards, including integrated circuit components packaged as surface mount devices.
Circuit boards of electronic equipment contain numerous integrated circuits and other electronic components soldered to the board. When components fail, electronics manufacturers need to remove and replace the components; components are also removed for recycling when boards become obsolete. However, the dense packing of circuit boards, and the small size of component pins, makes de-soldering of defective components and attaching or re-soldering new components difficult.
FIG. 1 shows a portion of a circuit board 10 (in section) on which an integrated circuit (IC) component 12 is mounted. The IC 12 is a surface mount technology (SMT) component of the ball grid array (BGA) type. BGA type ICs have ball-shaped electrical contacts 14 attached to the underside 16 of the IC. As shown in FIG. 2, the contacts 14 are arranged in a rectangular array comprising numerous rows and columns of contacts. The contacts provide an electrical connection between the IC and the circuit board. As shown in FIG. 1, a contact can be soldered to a plated-through hole 18 connected to a copper foil trace 20, providing an electrical path from the IC to the bottom of the circuit board. Usually the soldering is done by a wave soldering machine.
Once a BGA IC is soldered to a circuit board, the contacts 14 are inaccessible for later de-soldering or reworking. Re-soldering the component appears impracticable without wave soldering the entire board. Thus, many circuit board manufacturers do not attempt to de-solder, rework or re-solder BGA ICs; if a BGA IC fails, the whole board is discarded.
FIG. 3 shows a portion of a circuit board 10 (in section) on which an integrated circuit 22 is mounted. IC 22 is a standard SMT device or component having pins 24 protruding from the body 26 of the IC. Pins 24 are soldered to the top of circuit board 10 and do not extend into holes in the board. FIG. 3 is a simplified drawing; actual SMT devices may have 20 or more pins per side.
Once an SMT device is soldered to a board, de-soldering it or removing and re-soldering it is difficult because the pins are small (about 1 mm wide) and closely centered (0.040" centering is common). Applying sufficient heat to the pins, without damaging the IC itself with excessive heat, is difficult. For example, hot air (at approximately 650 degrees F.) can be directed at the pins, but the hot air inevitably flows over the component, posing a high risk of damaging or "cooking" the component or reflowing solder on adjacent components. In addition, hot air flow is not controllable so as to be useful for soldering. Alternatively, when heat is directly applied to the pins (also called leads) using a conventional soldering iron or equivalent, or through hot air, the component and circuit board act as a heat sink, absorbing much of the applied heat. As a result, the component quickly reaches a high temperature, de-soldering takes too long, and the component can be damaged before the solder melts.